Vulkan: readPixels improvement - use DynamicBuffer - Instead of using a new Staging image everytime, we now just use a DynamicBuffer that should be bit less wasteful. Bug: angleproject:2480 Change-Id: I09edfb16f6ec49a0ca320d9e5dd9ce34e9dbf99e Reviewed-on: https://chromium-review.googlesource.com/1059452 Commit-Queue: Luc Ferron <lucferron@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>